1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention relates to a dryer fabric, although it may find application in any of the fabrics used in the forming, pressing and drying sections of a paper machine, and in industrial process fabrics generally. Industrial process fabrics referred to herein may include those used in the production of, among other things, wetlaid products such as paper, paper board, corrugated paper board, and sanitary tissue and towel products; in the production of tissue and towel products made by through-air drying processes; in the production of wetlaid and drylaid pulp; in processes related to papermaking such as those using sludge filters, and chemiwashers; and in the production of nonwovens produced by hydroentangling (wet process), meltblowing, spunbonding, and airlaid needle punching. Such industrial process fabrics include, but are not limited to nonwoven fabrics; embossing, conveying, and support fabrics used in processes for producing nonwovens; and filtration fabrics and filtration cloths.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Contemporary fabrics are produced in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a woven or other type base fabric. Additionally, as in the case of fabrics used in the press section, the press fabrics have one or more base fabrics into which has been needled a batt of fine, nonwoven fibrous material. The base fabrics may be woven from monofilament, plied monofilament, multifilament or plied multifilament yarns, and may be single-layered, multi-layered or laminated. The yarns are typically extruded from any one of the synthetic polymeric resins, such as polyamide and polyester resins, used for this purpose by those of ordinary skill in the paper machine clothing arts.
The woven base fabrics themselves take many different forms. For example, they may be woven endless, or flat woven and subsequently rendered into endless form with a woven seam. Alternatively, they may be produced by a process commonly known as modified endless weaving, wherein the widthwise edges of the base fabric are provided with seaming loops using the machine-direction (MD) yarns thereof. In this process, the MD yarns weave continuously back-and-forth between the widthwise edges of the fabric, at each edge turning back and forming a seaming loop. A base fabric produced in this fashion is placed into endless form during installation on a paper machine, and for this reason is referred to as an on-machine-seamable fabric. To place such a fabric into endless form, the two widthwise edges are brought together, the seaming loops at the two edges are interdigitated with one another, and a seaming pin or pintle is directed through the passage formed by the interdigitated seaming loops.
Further, the woven base fabrics may be laminated by placing at least one base fabric within the endless loop formed by another, and by needling a staple fiber batt through these base fabrics to join them to one another as in the case of press fabrics. One or more of these woven base fabrics may be of the on-machine-seamable type. This is now a well known laminated press fabric with a multiple base support structure.
In any event, the fabrics are in the form of endless loops, or are seamable into such forms, having a specific length, measured longitudinally therearound, and a specific width, measured transversely thereacross.
Referring, now, more specifically to the dryer section, dryer cylinders are typically arranged in top and bottom rows or tiers. Those in the bottom tier are staggered relative to those in the top tier, rather than being in a strict vertical relationship. As the paper sheet being dried proceeds through the dryer section, it alternates between the top and bottom tiers by passing first. around a dryer cylinder in one of the two tiers, then around a dryer cylinder in the other tier, and so on sequentially through the dryer section.
In many dryer sections, the top and bottom tiers of dryer cylinders are each clothed with a separate dryer fabric. In dryer sections of this type, the paper sheet being dried passes unsupported across the space, or “pocket”, between the dryer cylinders of one tier and the dryer cylinders of the other tier.
As machine speeds are increased, the paper sheet being dried tends to flutter when passing across the pocket and often breaks. The resulting need to shut down the entire paper machine, and then to rethread the paper sheet through the dryer section, has an adverse impact on production rates and efficiency.
In order to increase production rates while minimizing disturbance to the paper sheet, single-run dryer sections are used to transport the paper sheet being dried at higher speeds than could be achieved in traditional dryer sections. In a single-run dryer section, a single dryer fabric follows a serpentine path sequentially about the dryer cylinders in the top and bottom tiers. As such, the paper sheet is guided, if not actually supported, across the pocket between the top and bottom tiers.
It will be appreciated that, in a single-run dryer section, the dryer fabric holds the paper sheet being dried directly against the dryer cylinders in one of the two tiers, but carries it around the dryer cylinders in the other tier. Alternatively, a single-run dryer section may have only one tier of dryer cylinders. Such a section has a turning roll, which may be smooth, grooved, or be provided with suction means, in the pocket between each pair of cylinders. This kind of dryer section is known as a single-tier dryer section.
Air carried along by the backside surface of the moving dryer fabric forms a compression wedge in the narrowing space where the moving dryer fabric approaches a dryer cylinder or turning roll. The resulting increase in air pressure in the compression wedge causes air to flow outwardly through the dryer fabric. This air flow, in turn, can force the paper sheet away from the paper contacting surface of the dryer fabric, a phenomenon known as “drop off”, when the paper sheet is not between the dryer fabric and the dryer cylinder. “Drop off” can reduce the quality of the paper product being manufactured by causing edge cracks, and can also reduce machine efficiency if it leads to sheet breaks.
Many paper mills have addressed this problem by machining grooves into the turning rolls with which the single-tier dryer fabric comes directly into contact or by adding a vacuum source to those turning rolls. Both of these expedients allow the air otherwise trapped in the compression wedge to be removed without passing through the dryer fabric.
In this connection, fabric manufacturers have also employed application of coatings to fabrics to impart additional functionality to the fabric, such as “sheet restraint methods.” The importance of applying coatings as a method for adding this functionality to, for example, dryer fabrics, has been cited by Luciano-Fagerholm (U.S. Pat. No. 5,829,488 (Albany), titled, “Dryer Fabric With Hydrophilic Paper Contacting Surface”). Luciano and Fagerholm have demonstrated the use of a hydrophilic surface treatment of fabrics to impart sheet-holding properties while maintaining close to the original permeability. However, this method of treating fabric surfaces, while successful in imparting sheet restraint, enhanced durability of the coating is desired. Thus, there stands a need to improve the wear properties of such coatings.
Turning now to the yarns used heretofore, particularly for dryer fabrics, monofilament yarns have typically been extruded with a simple circular cross-section. More recently, monofilaments with shaped cross-section have been produced. These shaped monofilaments have been used in woven fabrics to modify the fabric surface texture or density, or in particular, to control the fabric air permeability. The prior art includes U.S. Pat. No. 4,633,596 (Albany) which shows an inverted U-shaped polyester monofilament to be used in the fabrication of a forming wire to produce a desired smooth surface. However no filling of the opening to form a bicomponent filament is addressed. U.S. Pat. No. 5,097,872 uses an X-configuration cross-sectional monofilament in the machine direction yarns of a papermaking dryer fabric. In the weaving process, this monofilament is deformed to produce a smooth surface on the exposed paper side of the fabric while at the same time stability enhancing ridges are formed on the rear sides of these yarns. U.S. Pat. No. 4,216,257 refers to a U-shaped monofilament. The term “U-shaped” in this patent refers to the longitudinal, rather than the cross-sectional, shape of the monofilament. There are at least three Minnesota Mining and Manufacturing patents addressing this concept. U.S. Pat. No. 5,361,808 discloses yarns that are finned or T-shaped used as weft yarns. Note that the use of such yarns is said to broaden the permeability range. U.S. Pat. No. 5,998,310 shows monofilaments of a variety of cross-sections which may be distorted in the weaving process to achieve a number of effects. “Y” and “X” and “T” shaped monofilaments are described, but there is no mention of a U-shaped cross-section. U.S. Pat. No. 6,372,068 describes a thermoplastic monofilament bonded to a flat ribbon-like substrate to form a twist-tie for packages. U.S. Pat. No. 6,124,015 shows shaped portions of yarns for interlocking with each other.
Of particular interest is U.S. Pat. No. 5,888,915 (Albany) relating to fabrics constructed of bicomponent fibers. The bicomponent fibers have sheath and core materials with different melting points. When heated, the sheath/core yarns form a fused fabric structure which exhibits improved resistance to abrasion and also increased durability. None of the prior art however uses monofilaments having a U-shape which provides a receptacle for TPU, or which locks in or anchors a coating. All of the above referenced patents are however incorporated herein by reference.